Biodegradable resin foam

Abstract: A biodegradable resin foam made from biodegradable resin by expansion force due to vaporization of moisture caused by rapidly releasing fluidized biodegradable resin which is in a heated and pressurized environment and in which the moisture is trapped; said biodegradable resin comprising a combination of a first biodegradable resin ingredient having a melting point of 100°C or more and a second biodegradable resin ingredient having a melting point of 100°C or less.

Description: This invention relates to biodegradable resin foam. In general, synthetic resin has been applied to a variety of industrial fields because of exhibiting satisfactory mass productivity, moldability and durability. In particularly, synthetic resin foam is light-weight and exhibits increased cushioning properties, and is therefore widely used in various forms such as a protective casing for fragile articles such as glass products, cushioning material for packing, tableware, heat insulation material, sound insulation material and the like. However, this causes the amount of such synthetic resin products for disposal to be extensively increased, leading to various serious problems. More particularly, incineration of synthetic resin causes a large amount of harmful gas to be produced, leading to atmospheric pollution. Disposal of synthetic resin other than by incineration causes environmental pollution because it has resistance to oxidation and to decomposition by light and ozone. Also, synthetic resin has extensive intermolecular bonding, so that incineration causes generation of much heat, leading to damage to the furnace and therefore a decrease in its lifetime. In view of the foregoing, much attention has been recently directed to biodegradable resin and a great effort has been made to develop biodegradable resin. As a result, processing of biodegradable resin into a film material is now in the course of being put into practice. Also, development of foaming of biodegradable resin would lead to spread of applications thereof, to thereby permit advantages of biodegradable resin to be more widely exhibited. Techniques of foaming synthetic resin which have been carried out in the art include a method of producing foamed beads including the steps of charging styrene beads in a forming mold and adding water vapor thereto, followed by a decrease in pressure, a method of foaming synthetic resin by charging an extruder with, for example, styrene resin together with a foaming agent such as an organic solvent or the like to foam the resin due to a pressure reducing action occurring when the resin is extruded, and the like. However, such conventional chemical foaming techniques for foaming synthetic resin as described above fail to satisfactorily foam biodegradable resin due to a relationship between a softening point or melting point of the resin and a foaming temperature of a foaming agent and the like. Thus, there are known many problems which are encountered with techniques of foaming biodegradable resin to a high degree and forming the foamed resin. A first problem occurs when a biodegradable resin foam is to be produced by means of, for example, an injection molding machine used for production of a conventional synthetic resin foam. More particularly, when biodegradable resin fluidized due to heating and pressurization in a cylinder is extruded through a nozzle of the cylinder into a forming mold, to thereby be decreased in pressure, moisture in the resin is vaporized, leading to expansion. The moisture vaporized is then decreased in temperature, resulting in suspending in the form of steam in the mold or being condensed on an inner surface of the mold or an outer surface of a molded product. Biodegradable resin generally exhibits increased hygroscopicity, resulting in being readily softened and swollen when it is contacted wit moisture. In particular, a film of each of foamed cells on an outer portion of a molded or formed product is excessively decreased in thickness, so that it is readily softened when it absorbs condensed water. This results in the foamed cells being readily collapsed. Such collapse of the cells is also caused due to re-adhesion of moisture evaporated from the resin to the cells. The collapse causes portions of the formed product at which the cells are collapsed to be shrunk, leading to deformation of the formed product. When the formed product thus deformed is solidified, it is caused to be in a solid form substantially free of any foamed cell. Thus, the formed product thus obtained by injection molding fails to exhibit desired cushioning performance. A second problem is that the conventional chemical foaming techniques fail to provide a foamed product having a desired configuration and exhibiting a satisfactory cushioning function. More particularly, foaming of biodegradable resin is started upon release from a pressurized state, however, it is highly hard to reach the depths of a forming mold because of exhibiting increased viscosity when it is fluidized by heating. Therefore, foaming of biodegradable resin partially occurs before the resin extruded from the cylinder reaches the depths of the forming mold, resulting in a part of the resin which is to be foamed in the depths of the mold carrying out foaming in the middle of the mold, so that any cavity and/or void are formed in the foamed resin. Such a problem tends to occur in a forming mold of a complicated configuration, Thus, the so-formed biodegradable resin foam is pressedly forced by biodegradable resin subsequently extruded, so that the portion of the resin which carried out foaming on the way to the depths of the mold is crushed by the subsequently extruded resin. Thus, the prior art fails to form the foamed resin into a desired configuration. Also, the foamed resin fails to exhibit a satisfactory cushioning performance. A third problem occurs due to releasing of biodegradable resin fluidized by heating and pressurizing from a heated and pressurized environment. Release of the fluidized resin causes moisture contained in the resin to be vaporized and expanded, resulting in foaming of the resin, to thereby provide cells, during which the cells are decreased in temperature to a level of about 100°C due to vaporization of the moisture. This causes the cells to be somewhat shrunk and then solidified while being kept shrunk. Also, the cells are somewhat shrunk by water vapor surrounding the cells. Such cells are integrated together to form a foam. Thus, cavities and/or voids occur in the foam solidified, so that boundaries between the cells are discontinuous, to thereby cause the foam to be unsuitable for use for a cushioning material. A fourth problem encountered with the conventional chemical foaming techniques is caused during formation of an pressure reduced atmosphere. More particularly, when a heated and pressurized atmosphere in which biodegradable resin is placed is to be changed into a pressure reduced atmosphere, an evacuation or vacuum pump is generally used. Unfortunately, formation of such a pressure reduced atmosphere requires a considerable period of time, so that a pressure reducing action due to the change is rendered slow or inactive. Also, this causes moisture to re-adhere to cells while it is not fully exhausted, leading to softening of the cells, followed by collapse of the cells, resulting in the resultant resin foam being deteriorated in properties or quality. In order to prevent such re-adhesion of moisture to the cells, it would be considered that formation of the pressure reduced atmosphere is carried out using a large-sized vacuum pump or the like, resulting in time required for evacuation being reduced. However, this causes a significant increase in manufacturing cost of the foam. Further, injection of biodegradable resin from a nozzle of a cylinder of an injection molding machine into a forming mold arranged in a closed atmosphere causes injection resistance to be increased. In order to avoid the problem, it is required to arrange a large-sized injection molding machine. Unfortunately, this leads to an increase in cost of equipment and therefore manufacturing cost. In particular, in order to form a foam with high configuration accuracy, it is required to permit biodegradable resin to be spread throughout the forming mold. This is advantageously accomplished by keeping an atmosphere in the forming mold pressurized during injection molding. However, this causes injection resistance to be further increased, resulting in the above-described disadvantage being rendered amplified. EP-A-4356538 describes a biodegradable foamed-polyolefin resin in accordance with the preamble of claim 1 as well as its preparation, by heating and expanding a mixture of the resin and a water - containing natural cellulosic material. SUMMARY OF THE INVENTION The present invention has been made in view of the foregoing disadvantages of the prior art. It is an object of the present invention to provide a biodegradable resin foam which is substantially free of any discontinuous boundary between cells even when any cavity and/or void occurs in biodegradable resin foamed. In accordance with the present invention the biodegradable resin comprises a combination of a first biodegradable resin ingredient having a melting point of 100°C or more (or a main biodegradable resin ingredient) and a second biodegradable resin ingredient having a melting point of 100°C or less (or a low-melting biodegradable resin ingredient) and the melting point of said second biodegradable resin is less than the melting point of said first biodegradable resin. In a preferred embodiment of the present invention , the second biodegradable resin ingredient may be selected from the group consisting of polycaprolactone and a material containing polycaprolactone. In a preferred embodiment of the presentinvention, the biodegradable resin may have a substance selected from the group consisting of polyhydric alcohols and derivatives thereof added thereto. Thus, in the foam of the present invention the second biodegradable resin ingredient is kept from being immediately solidified, to thereby function as an adhesive with respect to the first biodegradable resin ingredient. Therefore, even when any cavity and/or void are unfortunately formed in the foamed biodegradable resin, cells of the foamed resin are permitted to adhere to each other through the second resin ingredient, resulting in the foam being provided with satisfactory quality. When the second biodegradable resin ingredient is selected from the group consisting of polycaprolactone and a material containing polycaprolactone, the function of the second biodegradable resin ingredient as an adhesive is substantially enhanced. Also, when the biodegradable resin has polyhydric alcohols and derivatives thereof added thereto, moisture in the resin is increased in boiling point, resulting in functioning also as a plasticizer, so that cells of the foamed biodegradable resin are rendered dense and uniform. The biodegradable resin starting material contains moisture and the biodegradable resin. For example , it may comprise the biodegradable resin and a hygroscopic fine-particle material having moisture absorbed therein and added to the biodegradable resin. Also, the biodegradable resin starting material may comprise moisture, the biodegradable resin and a water repellent material. The starting material of such composition ensures desired foaming of the resin and permits it to be finely and uniformly foamed. Also, the water repellent material may comprise a material which is not fully evaporated when the resin fluidized is released from the heated and pressurized environment. The material may include a natural fatty acid polymer. Use of the polymer as the water repellent material permits it to cover cells of the foamed resin to provide it with water repellent properties, to thereby prevent the cells from being collapsed due to contact with water. BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and many of the attendant advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings; wherein: Fig. 1 is a vertical sectional side elevation view generally showing an example of an apparatus suitable for use for practicing an embodiment of a method for producing a biodegradable resin foam according to the present invention; Figs 2(a) to 2(c) each are a schematic sectional view showing each of steps of the method of Fig. 1; Fig. 3 is a perspective view showing an embodiment of a biodegradable resin foam according to the present invention; Fig. 4 is a vertical sectional side elevation view showing an essential part of another example of the apparatus suitable for use for practicing the method of Fig. 1; Fig. 5 is a vertical sectional side elevation view showing an essential part of a further example of the apparatus suitable for use for practicing the method of Fig. 1; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Now, the present invention will be described hereinafter with reference to the accompanying drawings. The present invention is directed to a biodegradable resin foam, which is generally designated at reference character B in Figs. 2 and 3. The biodegradable resin foam B is produced by foaming biodegradable resin by means of expansion force due to vaporization of moisture. The biodegradable resin consists of a first biodegradable resin ingredient of 100°C or more in melting point (hereinafter also referred to as "main biodegradable resin ingredient") and a second biodegradable resin ingredient of 100°C or less in melting point (hereinafter also referred to as "low-melting biodegradable resin ingredient") and the melting point of said second biodegradable resin is less than the melting point of said first biodegradable resin. The term "biodegradable resin" used herein indicates a resin material which is decreased in physical properties due to a biological action and such biodegradable resin includes resin of the type that it per se is fully decomposed and resin of the type that it is blended with resin hard to be decomposed, resulting in it being provided with degradative properties. The resin of the former type includes products by microbes, products of natural high-molecular substances, products of petroleum materials and the like and that of the latter type includes blends with starch, blends with aliphatic polyesters and the like. Biodegradation of the biodegradable resin includes biodegradation by an enzyme such as lipase, amylase, cellulase, protease or the like, that by microbes in active sludge or the like, that by soil in a natural environment such as forest or cultivated land, and the like. More particularly, the biodegradable resin includes polyhydroxy butyric acid and derivatives thereof, pullulan, a cellulose-chitosan mixture, an ester compound of cellulose, amylose or wood meal, a polyester-nylon copolymer, and a blend of starch and polyethylene, as well as polyvinyl alcohol, polyether, polyurethane,, polyamide and the like. Such materials substantially have a low-melting point and are readily decomposed in the presence of water. The first biodegradable resin ingredient of more than 100°C or main biodegradable resin ingredient which is commercially available includes resin sold under "Mater-Bi" (registered trademark) from Nippon, Gosei Kagaku Kabushiki Kaisha, Japan. The resin was developed by Novamont belonging to a Montedison group, Italy and is said to be a thermoplastic biodegradable polymer comprising derivatives from a plurality of agricultural products such as starch and the like and denatured polyvinyl alcohol wherein the derivatives and alcohol get into each other at a molecular level, to thereby bonded to each other by hydrogen bonding. Also, it is said that the biodegradable resin is swollen due to absorption of moisture, resulting in biodegradation thereof being promoted, to thereby exhibit substantially the same biodegradability as paper in an environment in which microbes exist. In addition to "Mater-Bi" described above, for example, acetate which is an ester compound of cellulose may be of course used as the first biodegradable resin ingredient having a melting point of more than 100°C. The low-melting biodegradable resin ingredient, even when any cavity or void occurs in the biodegradable resin during formation of the biodegradable resin into a foam, functions to permit walls of cells of the foam to adhere to each other as much as possible. In the present invention, for example, polycaprolactone or a material containing polycaprolactone may be preferably used for the low-melting biodegradable resin ingredient. Such a low-melting biodegradable resin ingredient which may be commercially available includes resin sold under a tradename "Tone" from Nippon Unicar Kabushiki Kaisha, Japan. The commercially available resin "Tone" comprises polycprolactone which is aliphatic polyester chemically synthesized and is of the full decomposition type. Also, in the present invention, the biodegradable resin may have polyhydric alcohols and/or derivatives thereof added thereto as desired. In particular, glycols are preferably added to the resin. Addition of polyhydric alcohols or the derivatives permits moisture in the biodegradable resin to be increased in boiling point, so that the moisture functions also as a plasticizer. Also, the polyhydric alcohols and the derivatives per se each also act as a plasticizer, so that the cells of the foamed biodegradable resin may be rendered compact and uniform. The polyhydric alcohols and derivatives include glycerin, polyethylene glycol and the like, and derivatives thereof. A means for permitting moisture to be contained or present in the biodegradable resin is not limited to any specific means. For example, a suitable amount of moisture may be previously contained in the resin when pellets of the biodegradable resin for foaming are formed. Alternatively, a pretreatment step is provided for permitting a suitable amount of moisture to be positively contained in particles of the resin. Alternatively, moisture may be charged in a hopper together with biodegradable resin directly or through a kneader. Thus, it will be noted that feeding of the biodegradable resin and moisture is not limited to any specific manner. Also, addition of a fine particle-like hygroscopic material having moisture previously absorbed therein such as, for example, talc or silica to the biodegradable resin as in the pellets for foaming permits a foam having cells formed finely and uniformly to be provided, because moisture in the fine particles exhibits good compatibility and dispersion with respect to the biodegradable resin as compared with moisture directly added thereto and moisture in the fine particles contributes to foaming of the resin while originating from the fine particles. The biodegradable resin foam B of the present invention is produced by foaming the biodegradable resin kept fluidized by heating and pressuring by means of expansion force due to vaporization of moisture contained therein. At this time, the resultant foamed resin is reduced in temperature to a level of about 100°C by vaporization of the moisture. This causes the main biodegradable resin ingredient or first biodegradable resin ingredient to be solidified because a melting point thereof is 100°C or more, whereas the low-melting biodegradable resin ingredient or second biodegradable resin ingredient is kept from being solidified because its melting point is 100°c or less and the melting point of said second biodegradable resin is less than the melting point of said first biodegradable resin; so that the latter ingredient acts as an adhesive with respect to the former ingredient. Thus, even when any cavity and/or void occur in the foamed resin during the formation, walls between cells of the foamed resin are substantially adhered to each other, so that the biodegradable resin foam B may be provided with satisfactory quality. When the biodegradable resin starting material for the biodegradable resin foam B of the present invention comprises moisture, the biodegradable resin and a water repellent material, the water repellent material and moisture may be previously contained in or added to the biodegradable resin, followed by charging the starting material into an injection molding machine, because the biodegradable resin is placed under a heating and pressurizing condition in the presence of the water repellent material and substantial moisture. The water repellent material may include silicone compounds, fluorine compounds, waxes, polymers of a higher fatty acid and the like. In the illustrated embodiment, a commercially available material which is manufactured by Kabushiki Kaisha Sigma Gijutu Kenkyujo and sold from Tonen Kabushiki Kaisha under a tradename "Sigma coat" and mainly consists of polymers of a higher aliphatic acid and waxes may be used as the water repellent material. Addition of the water repellent material to the biodegradable resin may be carried out, for example, by diluting the water repellent material ("Sigma Coat") with water and then heating it to form an emulsion, followed by addition of the emulsion thereto. This permits the water repellent material and moisture to be concurrently added to the biodegradable resin. Alternatively, addition of the water repellent material to the biodegradable material and that of moisture thereto may be carried out separately from each other. For this purpose, the water repellent material may be added to the biodegradable resin containing moisture. The biodegradable resin foam B of the present invention thus formed is not limited to an application to a cushioning material. It may be directed to various applications such as a heat insulating material, a sound insulating material and the like as in a conventional foamed styrol material. Now, a method for producing a biodegradable resin foam according to the present invention will be described hereinafter together with an apparatus suitable for use for practicing the method. First, the apparatus will be described with reference to Fig. 1. The apparatus includes a cylinder 1 formed into a shape like a cylindrical container, which is provided on an upper portion of a rear end section thereof with a hopper 11 for charging a biodegradable resin starting material for a resin foam therethrough into the cylinder 1. The cylinder 1 is formed at a distal end thereof with a narrowed opening 1a and provided on around a periphery thereof with a heater 13. The cylinder 1 is provided therein with a screw 2 in a manner to extend in a longitudinal direction thereof and be in proximity to an inner surface thereof. The screw 2 is so arranged that a rear end thereof is outwardly projected through an opening formed at a rear end of the cylinder 1 and connected to a front portion of a hydraulic motor 21 for rotating the screw 2. The hydraulic motor 21 is mounted on a rear portion thereof with a piston 23, which is slidably received in an injection cylinder 22, so that the piston 23, hydraulic motor 21 and screw 2 are hydraulically moved integrally with each other in the longitudinal direction of the cylinder 1 and the screw 2 is rotated by the hydraulic motor 21. Thus, an injection machine S which is a so-called in-line screw type injection molding machine is constructed. The injection apparatus S is provided at a distal end section thereof with an air-permeable forming mold A, which includes a fixed mold part 3 formed with a cavity 31 communicating with the narrowed opening 1a of the cylinder 1 and a movable mold part 4 formed with a core 41. In the illustrated embodiment, the cavity 31 is formed into a configuration like a substantially triangular pyramid and the core 41 is formed into a triangular pyramid-like shape corresponding to the cavity 31 but smaller by a thickness of a formed product or resin foam B (Fig. 3) than the cavity 31. The fixed and movable mold parts 3 and 4 thus constructed are clamped together, to thereby define a space of a trihedral structure of which three surfaces are perpendicular to each other, resulting in providing such a formed product or biodegradable resin foam product shown in Fig. 3. Reference numeral 40 designates a mold clamp mechanism arranged in combination with the forming mold A, which mechanism includes a tie bars 42 and 43, and an actuator mechanism 44 for accessibly operating the tie bars 42 and 43 and movable mold part 4 so as to access the movable mold part 4 to the fixed mold part 3 while keeping both parts opposite to each other. The forming mold A is made of a porous material having a number of pores or holes formed therein while being arranged in a net-like manner so as to communicate an interior of the mold A with an exterior thereof. Such a porous material includes a foamed metal material, a sintered material made by sintering metal, ceramic or the like to which a filler capable of forming the material with voids is added, a material made by subjecting a wire mesh, a punched metal plate or the like to pressing and laminating, and the like. The forming mold A may be readily made, for example, by forming a single punched metal plate into a predetermined configuration. In this case, an increase in size of pores of the porous material leads to an increase in efficiency of exhaust of water vapor and efficiency of ventilation, however, an excessive increase in size of the pores causes roughness due to the pores to appear on a surface of the foamed resin after foaming of the resin. Thus, the pores of the porous material should be set to a size which permits a surface of the resin foam to be provided with smoothness suitable for applications to which the foam is directed while preventing resistance to permeation of air through the mold A from being excessively increased. In view of the above, a punched metal plate is suitable for use as the porous material for the forming mold A because it permits a diameter of the pores, the number thereof and a pitch thereof and the like to be readily set and is simplified in structure. The forming mold A and mold clamp mechanism 40 are arranged in a chamber 5 of an airtight structure acting as a pressure adjusting chamber. The chamber 5 includes a wall joined to a rear surface of the fixed mold part 3 of the mold A and is provided with a door (not shown) through which the resin foam made of biodegradable resin is removed. The chamber 5 has a pressure reducing pipe L 1 connected thereto, which is provided with a pressure reducing valve V 1 and connected to a pressure reducing pump P 1 through the valve 1 . Also, the chamber 5 has an atmospheric pressure releasing pipe L 0 connected thereto, which is provided with an atmospheric pressure releasing valve V 0 for releasing a pressure in the chamber from a reduced pressure state to an atmospheric pressure. Now, a method for producing a biodegradable resin foam according to the present invention practiced using the apparatus constructed as described above will be described hereinafter. First, the forming mold A is clamped through the mold clamp mechanism 40 and the pressure reducing valve V 1 is closed to keep the chamber 5 at an atmospheric pressure. Then, as shown in Fig. 1, the hopper 11 is fed with biodegradable resin particles 10 in which moisture is contained, which are then forcedly forwardly transferred in the cylinder 1 by means of the screw 2. The biodegradable resin particles 10 are increased in temperature to a level of a softening point of the resin or melting point thereof by shearing force due to rotation of the screw 2 and heating applied thereto from the heater 13, resulting in being fluidized in an inner space of the cylinder 1 defined on a distal end side of the screw 2. At this time, the inner space of the cylinder 1 is kept heated and pressurized, so that moisture contained in the biodegradable resin particles 10 is kept trapped in the fluidized biodegradable resin without evaporating therefrom. Then, rotation of the screw 2 is stopped and then the piston 23 in the injection cylinder 22 is actuated to forwardly move the screw as shown in Fig. 2(a), resulting in the biodegradable resin fluidized being injected through the narrowed opening 1a into a forming space R defined in the forming mold A at a stretch. This causes the biodegradable resin in the heated and pressurized state to be rapidly exposed to an atmospheric pressure, so that moisture trapped therein may be instantaneously vaporized, leading to foaming of the resin. Vaporization of the moisture causes expansion force to occur in the biodegradable resin. However, an outermost portion of the resin is contacted with an inner surface of the forming mold A, resulting in being regulated by a configuration of each of the cavity 31 and core 41, to thereby provide such a biodegradable resin foam product B as shown in Fig. 3 which is wholly integrally formed. In the foaming, the moisture is decreased in temperature after the vaporization and expansion, resulting in drifting inside and outside the forming mold A in a steam state or condensing on a surface of each of the forming mold A and foam product due to contact therewith, so that the moisture tends to form water droplets. In order to avoid formation of water droplets, the pressure reducing valve V 1 is open immediately after extrusion of the biodegradable resin into the forming mold A as shown in Fig. 2(b), to thereby reduce a pressure in the chamber 5 to a gauge pressure of, for example, about 750 mmHg by evacuation through the pressure reducing pump P 1 . During the evacuation, water vapor is partially sucked in the pressure reducing pipe L 1 prior to formation of water droplets. Also, moisture which tends to form water droplets on the surface of each of the forming mold A and resin form product B due to condensation is likewise removed by evacuation under a reduced pressure. Following evacuation under a reduced pressure for a predetermined period of time, the pressure reducing valve V 1 is closed and the atmospheric pressure releasing valve V 0 is open, so that air is introduced through the atmospheric pressure releasing pipe L 0 into the chamber 5 to return the chamber to an atmospheric pressure. Then, the screw 2 is retracted while being rotated as shown in Fig. 2(c), resulting in the next biodegradable resin fluidized starting to collect in the inner space of the cylinder 1 on the side of the distal end of the screw 2 for the next injection, during which cooling and solidification of the biodegradable resin foam product B in the forming space R of the forming mold A is completed. Thereafter, the forming mold A is rendered open to remove the formed biodegradable resin form product B therefrom, followed by being clamped for the next operation. The forming mold A, as described above, is formed at at least a part thereof of a porous material which permits an exterior of the mold A and an interior thereof to communicate with each other, to thereby be provided with air-permeability, resulting in a pressure of gas generated in the forming mold A due to expansion of moisture being effectively escaped to the exterior of the mold A. This not only effectively prevents the gas pressure from collapsing the resultant biodegradable resin foam, but decreases a pressure in the chamber 5 due to evacuation to forcibly remove moisture contributing foaming of the resin from the chamber 5, to thereby keep the resin foam from being contacted with the moisture. Thus, cells of the resin foam are substantially prevented from being collapsed, resulting satisfactory cells being uniformly distributed over the resultant resin foam, so that it may be effectively used as a uniform cushioning material. Alternatively, the illustrated embodiment may be constructed in such a manner that moisture is previously absorbed in a hygroscopic fine-particle material, which is then dispersed in the biodegradable resin, to thereby provide the biodegradable resin starting material for the resin foam. In this instance, the moisture is highly uniformly dispersed in the biodegradable resin fluidized which is extruded from the injection machine S while being kept carried on the fine particles, so that the moisture permits the resin to be foamed while acting the fine particles as an origin or center of the foaming, resulting in foaming of the resin being rendered uniform. The apparatus suitable for use for practicing the method of the present invention may be constructed as shown in Fig. 4. More particularly, an apparatus shown in Fig. 4 includes a chamber 5 divided into a movable-side part 51 and a fixed-side part 52, wherein the movable-side part 51 is arranged so as to be movable together with the movable mold part 4. Such construction permits the forming mold A and chamber 5 to be concurrently operated, to thereby facilitate removal of the biodegradable resin foam B from the mold A and chamber 5. Now, an experiment made for ascertaining functions and advantages of the present invention will be described hereinafter. Experiment Biodegradable resin foams were produced while varying a period of time between starting of extrusion of the biodegradable resin through the narrowed opening 1a and starting of evacuation under a reduced pressure (release lapse time) and a period of time during which evacuation was carried out (pressure reduction time), followed by observation of the products. Conditions for Experiment An injection molding machine of Type PS60E12ASE manufactured by Nissei Jushi Kogyo Kabushiki Kaisha, Japan was used as the injection machine S. The injection molding machine had a screw diameter of 36 mm, an injection rate of 12.3 cm/sec, an injection pressure of 1825 kgf/cm 2 , a screw rotation speed of 0 to 190 rpm at high torque and 0 to 250 rpm at low torque, and a back pressure of 60 kgf/cm 2 . The experiment was made under injection conditions that the injection rate, injection pressure, screw rotation speed, back pressure and variable are set to be 98%, 98%, 90%, 30% and 40 mm, respectively. The forming mold A used was constructed by forming a punched metal wholly formed with holes of about 1.2 mm in diameter and having a thickness of 1.5 mm into a box-like shape (100 mm x 100 mm x 50 mm) by welding and the like. The chamber 5 was formed into a volume 50 1 in such a manner as shown in Fig. 4 and evacuated to a gauge pressure of 99.9kPa (750mm Hg) Results of Experiment The results were as shown in Table 1. Release Lapse Time (sec) Pressure Reduction Time (sec) Volume (cc) Density (g/cc) State 0 5 635 0.041 Δ 5 0 454 0.050 X 5 10 504 0.046 ○ 5 20 502 0.046 ○ 5 30 526 0.043 ○ 5 120 531 0.043 ○ 1 30 608 0.042 o ○ In Table 1, X indicates that the resin foam exhibited remarkable shrinkage, Δ indicates that the resin foam was formed with a lot of voids, ○ indicates that the foam was decreased in shrinkage, and o ○ indicates that the foam was substantially free of shrinkage. The pressure reduction time "0" indicates that the chamber 5 was kept from being evacuated for pressure reduction, resulting in being kept at an atmospheric pressure. As will be noted from the results shown in Table 1, evacuation for pressure reduction permits shrinkage of the biodegradable resin to be decreased. In the illustrated embodiment, a timing of evacuation for pressure reduction in the chamber 5 may be set before, during or after extrusion of the biodegradable resin via the narrowed opening 1a into the forming mold A. Alternatively, the above-described pressure reduction due to evacuation of an atmosphere surrounding the forming mold A may be replaced with a means shown in Fig. 5. More particularly, the forming mold A is located in a ventilating duct D having a fan f arranged therein, so that moisture may be removed from an interior of the forming mold A by actuation of the fan f, to thereby keep the resin foam from being contacted with the moisture. Although narrowed opening 1a is kept open and the hopper 11 of the cylinder 1 is also kept open, the cylinder 1 is kept closed to a certain degree. However, a decrease in degree of closing of the cylinder 1 causes pressurization in the cylinder 1 to be insufficient, so that there is a possibility that a relationship between a softening point or melting point of the biodegradable resin and a boiling point of moisture or water pressurized in the cylinder causes the moisture to be partially vaporized, resulting in foaming of the biodegradable resin occurring in the cylinder 1 prior to injection of the resin into the forming mold. Also, this tends to cause foaming of the biodegradable resin extruded from the narrowed opening 1a while causing it to hang down from the narrowed opening 1a. In order to avoid such a problem, it is preferable that the illustrated embodiment is provided with a means for increasing a degree of closing of the cylinder 1. For this purpose, the narrowed opening 1a may be provided with a nozzle equipped with a shut-off valve. Alternatively, the hopper 11 for feeding the starting material to the cylinder 1 may be constructed so as to be tightly closed. Also, the hopper may be provided with a rotary valve. Further, foaming of the biodegradable resin in the cylinder 1 prior to the injection may be restrained by dissolving a nonvolatile solute such as polyethylene glycol in water and then adding the resultant solution to the biodegradable resin to increase a boiling point of water. When the hygroscopic fine particles described above is added to the biodegradable resin, production of the biodegradable resin foam B is not limited to extrusion of the biodegradable (2 resin from the injection molding machine or injection machine S into the forming mold A. It may be carried out in such a manner that the forming mold A in which the biodegradable resin having the hygroscopic fine-particle material previously dispersed therein is charged is arranged in a closed heating vessel and then the heating vessel is fed with heated water vapor, followed by a rapid decrease in pressure in the heating vessel. The hygroscopic fine-particle material is not limited to a round powdery configuration. It may be formed into a shape like a thin piece or a rod-like shape. In the illustrated embodiment, it is not necessarily required to add the hygroscopic fine-particle material to the biodegradable resin. The biodegradable resin may have water previously contained therein. Alternatively, biodegradable resin containing equilibrium moisture under an atmospheric pressure may be used for the present invention. Also, the biodegradable resin and moisture or water may be separately added to the hopper 1. In the illustrated embodiment, extrusion of the fluidized biodegradable resin into the forming mold may be carried out while placing a nozzle arranged with respect to the narrowed opening in the depths of the forming mold at the time when operation of the nozzle is started and retracting the nozzle relative to the forming mold during extrusion of the biodegradable resin. In the method described above, the water repellent material forms a film for covering each of foamed cells of the biodegradable resin foam, to thereby effectively prevent the foamed cells from being collapsed or broken due to swelling and/or softening of the biodegradable resin caused by contact of the resin foam with water. More particularly, formation of the film exhibits a first advantage of preventing moisture which has once contributed to foaming of the biodegradable resin due to vaporization and expansion thereof from re-adhering to the foam product and a second advantage of minimizing softening and swelling of the resin foam in use. Also, when a natural fatty acid polymer may be used as the water repellent material, the whole biodegradable resin foam exhibits satisfactory biodegradable properties because the polymer is likewise biodegradable. In the illustrated embodiment, the biodegradable resin particles 10, moisture and water repellent material are charged separately in the hopper 11. Alternatively, the cylinder 1 may be provided with an inlet port, through which the moisture or water repellent material may be fed directly to the cylinder 1. Also, the illustrated embodiment may provide a single biodegradable resin foam through the forming mold A by each batch production. Alternatively, the resin foam may be extruded through a cap, to thereby be obtained in the form of a continuous rod of a suitable sectional configuration corresponding to a configuration of the cap. In this instance, a plurality of the rod-like resin foams which have been extruded or is being foamed are combined with each other to provide a continuous rod-like product of an increased thickness. Further, an air-permeable cylinder in place of the forming mold A may be arranged in front of the narrowed opening 1a, so that a plurality of foam products extruded through the cylinder are joined in parallel to each other by means of an adhesive. Alternatively, the resin foams are joined together immediately after the formation, resulting in being adhered together by means of moisture on the surface. Thus, a single continuous resin foam may be provided. The present invention is not limited to foaming by extrusion through an injection molding machine or the like. For example, the present invention may be conveniently applied to foaming practiced according to a procedure wherein the air-permeable foaming mold A is arranged in a closed heating vessel, a predetermined amount of biodegradable resin containing a water repellent material is charged in the forming mold A, heated water vapor is introduced into the heating vessel and then a pressure in the heating vessel is reduced. Also, in the present invention, the step of foaming the biodegradable resin and the step of subjecting the foamed resin to formation may be carried out separately. As can be seen from the foregoing, the biodegradable resin foam of the present invention comprises a combination of the main biodegradable resin ingredient or first biodegradable resin ingredient having a melting point of 100°C or more and the low-melting biodegradable resin ingredient or second biodegradable resin ingredient having a melting point of 100°C or less and the melting point said second biodegradable resin is less than the melting point of said first biodegradable resin. Thus, in the resin foam, the second biodegradable resin ingredient is kept from being immediately solidified, to thereby function as an adhesive with respect to the first biodegradable resin ingredient. Thus, even when any cavity and/or void are generated in the foamed biodegradable resin, cells of the foamed resin are permitted to adhere to each other, resulting in the biodegradable resin foam being provided with satisfactory quality. In particular, when the second biodegradable resin ingredient is selected from the group consisting of polycaprolactone and a material containing polycaprolactone, the function of the second biodegradable resin ingredient as an adhesive is significantly enhanced. Also, when the biodegradable resin has polyhydric alcohols and derivatives thereof added thereto, moisture in the biodegradable resin is increased in boiling point, resulting in functioning also as a plasticizer, so that cells of the foamed biodegradable resin are rendered dense and uniform. While preferred embodiments of the invention have been described with a certain degree of particularity with reference to the drawings, obvious modifications and variations are possible in light of the above teachings. It is therefore to be understood that the invention may be practiced within the scope of the appended claims.

Claims: A biodegradable resin foam made from biodegradable resin by expansion force due to vaporization of moisture caused by rapidly releasing fluidized biodegradable resin which is in a heated and pressurized environment and in which the moisture is trapped; characterised in that said biodegradable resin comprises a combination of a first biodegradable resin ingredient having a melting point of 100°C or more and a second biodegradable resin ingredient having a melting point of 100°C or less and the melting point of said second biodegradable resin is less than the melting point of said first biodegradable resin. A biodegradable resin foam as defined in claim 1, characterized in that said second biodegradable resin ingredient is polycaprolactone or a material containing polycaprolactone. A biodegradable resin foam as defined in claim 1 or claim 2, characterized in that said biodegradable resin has a polyhydric alcohol or derivative thereof added thereto.

Inventors: NAKANISHI MOTOYASU, NAKANISHI, MOTOYASU
Applicants: SUZUKI SOGYO KK, SUZUKI SOGYO CO., LTD.
Application year: 1994
Main classification: B29B17
IPC codes: C08J9, B29C44